Fundamental to the acquisition of communication skills is the normal development of hearing. At the neural level, the development of hearing requires the proper development of the sensory receptor cells in the inner ear (inner and outer hair cells) and their innervation by afferent and efferent fibers. Presently, the molecular and morphological processes that lead to the differentiation of this neural system are poorly understood. Thus, we propose to initiate a series of investigation to begin to understand those processes. The goal of this research is to comprehend the molecular and morphological development of the efferent innervation in the organ of Corti of the mouse. Recent studies suggest that the efferent system may consist of discrete neuronal subunits that differ functionally depending on heir biochemical characteristics. We propose, therefore, to use biochemical, histochemical and immunocytochemical techniques to study the expression of the enzymes of acetylcholine metabolism - choline acetyltransferase (ChAT), AchE and the molecular forms of AChE - and the neuropeptide neurotransmitters methionine-enkepalin and calcitonin gene-related pepteide in relation to the ultrastructural development of efferent fibers in the cochlea. Our data suggests that biochemical expression of the efferent fibers preceed their morphological differentiation. Thus, we expect these techniques to provide a sensitive probe for the early detection of efferent fibers to study the patterns of their growth and differentiation. Futhermore, our evidence of the molecular diversity of the cholinergic system along the cochlear spire suggest functional differences in their synaptic apparatus. The data stress a need for selective immunological visualization to distinguish the molecular neuronal subsystems. We expect that by the means of monoclonal antibodies to different AChE molecular forms (globular 6/4S, 10S and asymmetric 16S) we will be able to determine the developmental time and the topography of different subsystems during the morphological differentiation of the efferent system.